Method and apparatus for measuring the impedance of an electrical energy supply system

ABSTRACT

The invention relates to a method and an apparatus for measuring the impedance of an energy supply system at a rated frequency by impressing a testing current ( 3 ) into the system and measuring ( 4 ) the resulting changes in a system current and a system voltage. According to the invention, the system current and the system voltage are measured during a first preselected time interval (T 1 ) without impressing of the testing current and are measured during a second preselected time interval (T 2 ) with the impressing of the testing current. The testing current is composed of at least one periodic signal with at least one testing frequency (f 1,  f 2 ) that deviates from the rated frequency. Based on the measured system currents and system voltages, a Fourier analysis is used to determine a resulting impedance for the testing frequency (f 1,  f 2 ). The impedance of the energy supply system at the rated frequency is derived from the impedance obtained for the testing frequency (f 1,  f 2 ) (FIG.  1 ).

TECHNICAL FIELD

[0001] The invention relates to a method and an apparatus for measuringthe impedance of an electric energy supply system, i.e. an electricityutility (public) grid or system, at a rated (nominal) frequency byimpressing a testing current into the grid or system and measuring theresulting changes in the system current and the system voltage.

BACKGROUND OF THE INVENTION

[0002] In the operation of electricity utility (or public) grids,hereinafter also called electrical energy supply systems or simply“systems”, it is occasionally desirable to be able to switch off theentire system or parts thereof for all consumers or individual consumersor to maintain a switched-off or disconnected condition after a failureor breakdown (loss of mains) in order to assure a voltage-free state forcarrying out maintenance or repair work. This is not easily possible ifthe grid or system is supplied not only by a central energy supplyplant, but also by decentralized voltage sources such as photovoltaicplants, since even in the event that an operator of the central plantexecutes a system shutdown or the like, these decentralized voltagesources continue to supply energy into the system and therefore lead toa local island formation (islanding).

[0003] When there are decentralized voltage sources, which, in the eventof a system disconnection or the like, are not sufficient to cover theexisting energy demand and therefore in particular suffer a drop involtage, an undesirable island formation can be avoided by monitoringthe voltage and automatically shutting down the relevant decentralizedplant if the voltage falls below a predetermined minimum voltage.However, if the decentralized voltage source is in a position tocontinue maintaining the system voltage within strict tolerances withregard to amplitude and frequency even in the event of a partial ortotal outage or shutdown of essential components, then theabove-described monitoring method alone would not be able to cause thedecentralized voltage source to shut down.

[0004] In order to prevent local islanding from being produced andposing a threat to people and/or electrical systems even in such a case,methods and apparatuses of the type described at the beginning havebecome known. They serve the purpose of detecting unusual impedancechanges in the public grid and shutting down associated decentralizedvoltage sources when critical values are reached. As a rule, a shutdownis required only if an impedance increase of 0.5 Ω or more (VDE 0126)occurs in the system. It is assumed that such impedance changes do notoccur during normal system operation and are therefore characteristic ofthe outage or the desired shutdown of a transformer station or the like.A further requirement is that the shutdown occurs within a period of atmost five seconds after the shutdown or outage of the system. This makesa continuous or quasi-continuous monitoring and measurement of thesystem impedance necessary.

[0005] With the use of known methods and apparatuses of the typesmentioned at the beginning, either high and/or pulse-shaped testingcurrents with system frequency are fed into the system or testingimpedances in the form of capacitors or transformers are connected intothe system in rapid succession in order to obtain information from thethus modified system currents and system voltages about the systemimpedance, which is usually dependent on the frequency (DE 36 00 770 A1,DE 195 04 271 C1, DE 195 22 496 C1, DE 100 06 443 A1). However, suchmethods and apparatuses are accompanied by a variety of undesirableproblems. For example, if it is necessary to use a comparatively high,possibly pulse-shaped testing current in order to analyze the systemvoltage with regard to its base or fundamental frequency or its harmonicoscillations, then a powerful interference is disadvantageously producedin the system voltage, which can lead to an erroneous interpretation ofmeasurement results. On the other hand, the use of a test impedance, forexample, requires a costly set of power electronics. Finally, ifbroadband testing currents are provided, then it is necessary to analyzethe system voltage over a larger frequency spectrum, thus requiring theprovision of substantial computing power and therefore high-qualitymicroprocessors or digital signal processors. Apart from this, abroadband testing current causes so many undesirable interferences inthe system that it is hardly possible to distinguish the expected testresponse from among these interferences.

SUMMARY OF THE INVENTION

[0006] Based on this prior art, an object of the present invention is toembody the method and apparatus described above in such a way that themeasurement of the system impedance generates only slight interferencesin the system.

[0007] A further object of the invention is to provide a method and anapparatus for measuring the impedance of an electrical energy supplysystem in such a manner that the impedance can be determined withsufficient accuracy even when there are interferences in the systemvoltage and when the system frequency is not constant.

[0008] Yet a further object of the present invention is to provide amethod and an apparatus which make possible an automatic measurement ofthe impedance of an electrical energy supply system with in a reliable,easy and unexpensive manner.

[0009] The method according to this invention comprises the steps ofmeasuring said system current and said system voltage during a firstpreselected time interval (T1) without the impressing of said testingcurrent and during a second preselected time interval (T2) with theimpressing of said testing current, composing said testing current of atleast one periodic signal with at least one testing frequency (f1, f2)that deviates from said rated frequency, using, based on said measuredsystem currents and system voltages, a Fourier analysis to determine aresulting impedance for said testing frequency (f1, f2) and derivingsaid impedance of said energy supply system at the rated frequency fromsaid impedance obtained for said testing frequency (f1, f2).

[0010] The apparatus of this invention for measuring the impedance of anenergy supply system at a rated frequency comprises means for impressinga testing current into said system, a measuring device for measuring asystem current, a system voltage and changes in said system current andsaid system voltage caused by the impressing of said testing current,and an evaluation unit connected to said measuring device, wherein saidsystem current and said system voltage are measured during a firstpreselected time interval (T1) without the impressing of said testingcurrent and during a second preselected time interval (T2) with theimpressing of said testing current, said testing current is composed ofat least one periodic signal with at least one testing frequency (f1,f2) that deviates from said rated frequency, a Fourier analysis is used,based on said measured system currents and system voltages, to determinea resulting impedance for said testing frequency (f1, f2) and theimpedance of said energy supply system at the rated frequency is derivedfrom the impedance obtained for said testing frequency (f1, f2).

[0011] The invention has the advantage that due to the measurement ofthe system voltage and system current with at least one frequency thatdeviates from the system frequency, for example with an intermediateharmonic or interharmonic frequency, the influences of interference inthe system voltage and the system frequency are to a large extentreduced. In addition, comparatively low testing currents that arecontinuous during the testing interval can be used, which do not causeany interference in the system or cause any undesirable harmonicoscillations. It is also possible to largely integrate the components,which are required to carry out the method according to the invention,into a voltage source that is parallel to the system and is presentanyway, which permits the apparatus according to the invention to beembodied in a particularly inexpensive fashion overall.

[0012] Other advantageous features of the invention ensue from thedependent claims.

[0013] The invention will be explained in detail below through exemplaryembodiments in conjunction with the accompanying drawings.

[0014]FIG. 1 shows an exemplary embodiment of the apparatus according tothe invention, in the form of a simplified block circuit diagram;

[0015]FIG. 2FIG. 2 shows a view similar to the one shown in FIG. 1 of aparticularly inexpensive variant of the apparatus according to theinvention, which uses components that are present anyway e.g. in astatic converter; and

[0016]FIG. 3 schematically depicts the progression or characteristic ofthe system voltage and the system current during a complete testinginterval using the method according to the invention.

[0017]FIG. 1 schematically depicts a conventional electrical energysupply system or electricity public grid (e.g. 230 V_(eff), 50 Hz) witha central alternating current source 1 and an impedance 2 with the valueZ_(N), which among other things, is due to a number of consumers, notshown, being connected to the system in parallel. According to FIG. 1,the apparatus according to the invention includes a controllablealternating current source 3, which can also be connected into thesystem at a system connection, and which can impress a testing currentinto the system, as well as a measuring device 4 with two inputs 5 and6, and an evaluation unit 7. The inputs 5 and 6 of the measuring device4, which are connected to the system in a suitable manner, are used inan intrinsically known manner to detect and preprocess measurementvalues for the system current (input 5, arrow I) and the system voltage(input 6, arrow U). A “preprocessing” of the obtained measurement valuesis understood here, for example, to mean that the measured voltage offor example 325 V is reduced to a value suitable for measurement,processing, and control purposes, for example 10 V, is galvanicallyseparated from the system for safety reasons, for example with opticalor magnetic means, is filtered as needed, and is converted to a digitalvalue—possibly with the aid of an analog/digital converter—if an analogprocessing of the measurement signals is undesirable. The measurementvalues obtained for the system current can be processed in acorresponding manner. Finally, two outputs 8 and 9 of the measuringdevice 4 emit measurement signals, which are characteristic for themeasured system currents and system voltages. These outputs 8 and 9 arealso connected to the evaluation unit 7 in which the receivedmeasurement signals can be evaluated in accordance with the methodaccording to the invention.

[0018] The method according to the invention begins with measuring thesystem current I and the system voltage U during a first preselectedtime interval T1, which is indicated in FIG. 3. A thin, dotted lineindicates the system current I, whereas a thick, solid line indicatesthe system voltage U. A customary frequency of 50 Hz has been assumed,although in a corresponding modification, the invention can naturallyalso be used in other systems, e.g. with 60 Hz. At the beginning of asecond preselected time interval T2 (FIG. 3), the controllable currentsource 3 then additionally impresses a testing current into the system,for example by virtue of an output 10 of the evaluation unit 7 sending astart signal, which initiates the impressing of the testing current. Inthe exemplary embodiment, the testing current is synchronously switchedon and off again during a respective zero crossover of the systemvoltage and the system current, respectively. The testing current in theexemplary embodiment is composed of two periodic signals with thefrequencies f1=40 Hz and f2=60 Hz, both of which deviate from the 50 Hzrated frequency of the system, thus causing the changes in the currentand voltage progression or course that are shown in an exaggeratedfashion in the time interval T2 in FIG. 3.

[0019] The measuring device 4 measures the currents and voltages duringthe two time intervals T1 and T2, which amount to a complete measurementcycle. These measurement values are used to carry out a Fourier analysisin the evaluation unit 7 in order, based on both the measurement valuesobtained without the testing current in time interval T1 and themeasurement values obtained with the testing current in time intervalT2, to determine the resulting impedance Z_(N) of the energy supplysystem at the rated frequency. To this end, the method proceeds asfollows:

[0020] First, the system current and the system voltage are measuredduring the time interval T1 and these are used to calculate the valuesI_(T1) (t) and U_(T1) (t). Then, (FIG. 3), the testing current isimpressed with the frequencies f1 and f2, wherein

I_(test)=I_(f1)·sin(2πf1t)+I_(f2)·sin(2πf2t),

[0021] which yields measurement values of the system current and thesystem voltage I_(T2) (t) and U_(T2) (t).

[0022] In the evaluation unit 7, a discrete Fourier transformation DFTis carried out, in which first, complex Fourier coefficients of thesystem current and the system voltage are determined in accordance withthe following equations:

DFT(U_(T1)(t),f)=U_(T1)(f)=Re(U_(T1)(f))+j·Im(U_(T1)(f))

DFT(I_(T1)(t),f)=I_(T1)(f)=Re(I_(T1)(f))+j·Im(I_(T1)(f))

DFT(U_(T2)(t),f)=U_(T2)(f)=Re(U_(T2)(f))+j·Im(U_(T2)(f))

DFT(I_(T2)(t),f)=I_(T2)(f)=Re(I_(T2)(f))+j·Im(I_(T2)(f)).

[0023] In these equations, “Re” stands for the real components and “Im”stands for the imaginary components. In addition, the calculation iscarried out separately for the two frequencies f1 and f2 and, as isapparent, is carried out separately for the time intervals T1 and T2,thus yielding a total of eight real components and eight imaginarycomponents.

[0024] Based on the Fourier coefficients, a subtraction operationexecuted according to the following equations is then used to calculatethe complex differences for the time intervals T1 and T2, which arecharacteristic for the changes in the system current and system voltage:$\begin{matrix}{{U_{DIFF}(f)} = {{{Re}\left( {U_{T2}(f)} \right)} - {{Re}\left( {U_{T1}(f)} \right)} + {j\left\lbrack {{{Im}\left( {U_{T2}(f)} \right)} - {{Im}\left( {U_{T1}(f)} \right)}} \right\rbrack}}} \\{{I_{DIFF}(f)} = {{{Re}\left( {I_{T1}(f)} \right)} - {{Re}\left( {I_{T2}(f)} \right)} + {{j\left\lbrack {{{Im}\left( {I_{T1}(f)} \right)} - {{Im}\left( {I_{T2}(f)} \right)}} \right\rbrack}.}}}\end{matrix}$

[0025] Here, too, separate procedures are used for the frequencies f1and f2, thus yielding four respective real and imaginary components.These can be used in the following manner to calculate the complexsystem impedances: ${{Z_{N}(f)} = \frac{U_{DIFF}(f)}{I_{DIFF}(f)}},$

[0026] which separately yields a respective value Z_(N) (f1) and Z_(N)(f2) for f1 and f2, from which the resulting system impedance at thesystem frequency f_(SYSTEM) is derived by averaging:${Z\left( f_{SYSTEM} \right)} = {\frac{{Z_{N}({f1})} + {Z_{N}({f2})}}{2}.}$

[0027] In the exemplary embodiment, it is therefore possible to achievea sufficiently precise calculation of the system impedance by averaging,particularly because the two testing frequencies f1, f2, for example 40Hz and 60 Hz, are preferably selected to be symmetrically situated oneither side of the system frequency f_(SYSTEM). Alternatively, thetesting frequencies can also be selected, for example, as f1=45 Hz andf2=65 Hz or f1=10 Hz and f2=100 Hz. In these cases, instead of using anaveraging, the resulting system frequency can also be derived, forexample, by means of a straight approximation based on the impedancevalues obtained for f1 and f2. It would be analogously possible tocalculate the testing current from only a single periodic signal withonly one frequency that deviates from the system frequency. In thiscase, the Fourier analysis could be carried out in a correspondingmanner, wherein the equations given above would only have to be solvedfor one frequency each. In addition, only one value${Z_{N}({f1})} = \frac{U_{DIFF}({f1})}{I_{DIFF}({f1})}$

[0028] would be obtained, which would render averaging and straightapproximation impossible. However, since the complex impedances andtherefore also the frequency progressions of the impedances areobtained, these can be used to calculate the impedance at the ratedfrequency.

[0029] In the description of the exemplary embodiment, it has also beenassumed that sinusoidal testing currents are impressed and that thetesting current is composed of sinusoidal signals. Even if being thebest way however, this is not absolutely required because a triangulartesting current, for example, could also be impressed, which for itspart can be thought of as being comprised of a number of sinusoidalsignals with various frequencies and therefore permits a Fourieranalysis with one or more of these frequencies.

[0030] Each of the embodiments cases described above involves theadvantage intended by the invention that even with the presence offrequency-dependent impedances, it is possible to determine the systemimpedance at the rated frequency of the system by first selectivelydetermining impedances at frequencies not present in the system and fromthese, calculating the system impedance at the rated frequency by meansof an approximation. For the respective testing frequencies used here,intermediate harmonic frequencies are preferably selected which also donot occur in the harmonic oscillations associated with the systemfrequency. This produces a determination method, which is largely freeof interferences in the system and also yields sufficiently preciseresults particularly with the use of testing frequencies that are closeto the rated frequency.

[0031] In order to be able to carry out an exact Fourier analysis, thetime intervals T1 and T2 are preferably identical and are selected tobegin and end with the same phase position so that the same phasepositions for the phases can be achieved with and without the testingcurrent. This is possible if T1 and T2 are at least as long as thelowest common multiple of the periods of the system frequency and thetesting frequency or frequencies and therefore each of these periodsoccurs at least once in the time interval T2. At the system frequency of50 Hz and testing frequencies of 40 Hz and 60 Hz, the followingequations apply:

T_(SYSTEM)=1/f_(SYSTEM)=20 ms

t1=1/f1=25 ms

t2=1/f2=16.667 ms,

[0032] from which it follows that the lowest common multiple in thisinstance is 100 ms (see FIG. 3). If t1=1/f1=22.222 ms (f1=45 Hz) andt2=1/f2=15.385 ms (f2=65 Hz), then this would yield a lowest commonmultiple of 200 ms, for example. In order to produce reasonable valueshere, the invention proposes selecting testing frequencies that liebetween 9 Hz and 121 Hz.

[0033] Since the shutdown of the decentralized voltage sources shouldoccur at the latest within five seconds of the system shutdown or systemoutage, the second time interval T2 is preferably no longer than 1second. T1 can be selected to be correspondingly short so that a totalmeasuring time of maximally 2 seconds is required. Actually, however, inthe exemplary embodiment 200 ms is sufficient in order, when theevaluation unit 7 detects an impedance change of 0.5 Ω or more in thesystem, to generate a signal that can be used to shut down adecentralized voltage source and that can be emitted, for example, froman output 11 (FIG. 1) of the evaluation unit 7. It would therefore besufficient to run through the 200 ms long measuring cycle once everyfive seconds.

[0034] The signal appearing at the output 11 can be generated in variousways. In the simplest case, the signal is a testing signal, whichindicates the value for the system impedance determined in theevaluation unit 7, and a device associated with the decentralizedvoltage source checks whether or not the voltage source must be shutdown. Alternatively, the signal can be a status signal, which indicateswhether the system impedance is greater or less than a criticalthreshold value. In this case, the voltage source can be associated witha shutdown device that reacts to the status signal. Another possibilityis comprised of generating a switching signal at the output 11, whichdeactivates the decentralized voltage source or disconnects it from thesystem in the event that the system impedance change has exceeded acritical threshold value of 0.5 Ω, for example. The particular manner inwhich the desired shutdown of the decentralized voltage source isexecuted can be made to depend on the requirements of the individualcase.

[0035] The transmission of signals from the measuring device 4 to theevaluation unit 7 or from this evaluation unit 7 to the controllablecurrent source can take place in an analog or digital fashion asnecessary.

[0036] The apparatus described in conjunction with FIG. 1 is preferablyproduced in the form of a compact apparatus contained in a housing, asindicated by the line 12 encompassing it. In this instance, the outsideof the housing is only provided with the connections, which are requiredto connect the current source 3 and measuring device 4 to the system,and with the output 11, which is designated for connection to a voltagesource to be monitored and controlled.

[0037]FIG. 2, in which same parts are provided with the same referencenumerals as in FIG. 1, shows an embodiment which is considered to be thebest one up to now. A static current (power) converter 14 connected tothe system in parallel, is connected to a decentralized direct-currentsource 15 such as a photovoltaic system, converts the direct currentgenerated by this direct current source into alternating current, andfeeds this current into the system. Current converters 14 of this kindcan be thought of not only as controllable current sources, but also asa rule have means for detecting current or voltage measurement values, aset of power electronics, an electronic control unit 16 for theseelectronics, and means for regulating the amplitude and frequency of thecurrent that is fed into the system. In such a case, therefore,components or structural groups, which are in principle already presentin the current converter 14, can be used to produce the apparatusaccording to the invention.

[0038] As indicated in FIG. 2, the current source 3 according to FIG. 1,for example, is replaced by the current converter 14 according to FIG. 2so that the apparatus according to the invention contains only themeasuring device 4 and evaluation device 7, whose output 11 is connectedto the control unit 16 of the current converter 14. Apart of this, theapparatus is contained in a housing 17, for example, and functions inthe same way as the one in FIG. 1.

[0039] In the embodiment of FIG. 2, the testing current is impressedinto the system by virtue of a testing current being superposed onto thebase current generated by the current converter 14, which is suppliedwith electrical energy by the direct-current source 15. Thesuperposition can occur, for example, by virtue of a desired value forthe testing current being superposed onto the operationally requirednominal current value of the current converter 14. During the timeinterval T2, therefore, the current converter 14 supplies the systemwith a base current, which is modified by the testing current.Therefore, the desired value for the testing current can be stored, forexample, in an electronic memory of the current converter 14, or can begenerated by this current converter 14 in some other way and beactivated during the time interval T2. The activation can occur with theaid of a switching signal, which is emitted by the evaluation unit 7,indicates the beginning and end of the time interval T2, and appears ata second output 18 of the evaluation unit 7, for example.

[0040] In order for the changes in the system current and the systemvoltage caused by the testing current to experience the least possibleinterference due to the events occurring in the system, the measuringdevice 4 preferably contains either a band-stop filter matched to therated frequency of the system or band-pass filters matched to thetesting frequencies f1, f2. This largely prevents interference due tosignals with frequencies other than the testing frequencies f1, f2.

[0041] According to a particularly preferred modification of theinvention, a known testing impedance 19 (FIG. 2) with the value Z₁ isconnected between the controllable voltage source 3 or 14 and thesystem, which testing impedance is connected in parallel with a switch20 that can be controlled or switched on and off. A control input ofthis switch 20, for example, is connected to a third output 21 of theevaluation unit 7. The purpose of the test impedance 19 is to test thefunction of the apparatus according to invention from time to time.

[0042] Essentially, this proceeds as follows:

[0043] When the switch 20 is closed, the test impedance 19 isshort-circuited. In this case, the apparatus operates in theabove-described manner by measuring the system impedance 2. However, ifthe switch 20 is opened by means of a control signal sent by theevaluation unit 7, then the above-described apparatus measures the sumof the impedances 2 and 19, i.e. the value Z_(N)+Z₁. Since Z₁ is known,this measurement can be used as needed to test whether the testimpedance 19 is accompanied by the expected Z₁ impedance jump and theapparatus is therefore operating properly.

[0044] In the above described exemplary embodiments, it was assumed thatthe control signals for the beginning and end of the testing current andfor turning the switch 20 on and off are generated and sent directly bythe evaluation unit 7. Alternatively, however, it would naturally alsobe possible to provide a control unit especially for this purpose.

[0045] Another preferred exemplary embodiment of the invention providesfor an individual identification, e.g. a serial number or the like, tobe associated with each device to which the above description applies.The identification is embodied so that when the testing current of thisdevice is impressed, this identification is automatically fed along withit into the system and can then be recognized by other devices that arealso connected to the system. Such an identification can be achieved,for example, by superposing a current with a very high frequency ontothe testing current generated by the associated device; this highfrequency is different for each device and does not interfere witheither the desired measurement of the system impedance or the Fourieranalysis carried out for this purpose. Alternatively, the apparatuscould also be embodied in such a way that each device feeds a currentflow, which is characteristic for the respective identification, intothe system. The manufacturer can predetermine the identification foreach individual device by storing it e.g. in a memory of the evaluationunit 7, by setting it by means of an encoded switch position, or byassociating it in some other way. Correspondingly, in this case, themeasuring device 4 or the evaluation unit 7 of each device is providedwith a recognition device, which checks and analyzes the measured systemcurrent for the presence of such an identification.

[0046] If several decentralized voltage or current sources 3 or 14 areconnected to the system in parallel, then the use of the above-describedidentifications and an additional simple control unit makes it possibleto ensure that only a single device or a preselected number ofassociated devices impresses a testing current into the system at aparticular time. This can prevent an unknown number of testing currentsfrom being impressed into the system at any given time and thusgenerating false measurement results. In this connection, it would bepossible, for example, to program the device so that a measurement cycleto be initiated by it and in particular, the time interval T2 to beinitiated by it are enabled only if no other device is simultaneouslyimpressing a testing current, or so that it suppresses the enabling ofits own testing current as long as the testing currents of other devicesare being impressed into the system. In other words, it is possible foreach device, in the evaluation of its own impedance calculation, to takeinto account whether the testing currents of other devices have alsobeen activated at the same time. Since the time intervals T1 and T2according to the exemplary embodiment above are only 100 ms each, but aperiod of five seconds, for example, is available for shutting down thedecentralized voltage source 15, theoretically twenty-five devices couldbe operated without two or more devices having to output testingcurrents at the same time. A particular advantage of the above-describedprocess also lies in the fact that the various current sources can beoperated independently of one another and without an overriding timecontrol or the like.

[0047] The invention is not limited to the exemplary embodimentsdescribed, which can be modified in numerous ways. In particular, theabove-described apparatuses can be used for monitoring and/or shuttingdown system suppliers other than the photovoltaic ones described. Asshown in FIGS. 1 and 2, in principle, it makes no difference whether thevarious components or functions of the apparatuses are totally or inpart already present in the respective system supplier or have to beadditionally incorporated into or implemented in them. Also, the networkimpedance calculating method used in the individual case can bedifferent from the one described, to which end each evaluation unit 7 issuitably equipped with the required computers (microcontrollers,processors, or the like) and the associated software. It is also clearthat the methods and apparatuses described in conjunction with asingle-phase system can be used analogously in three-phase systems byvirtue of each phase being associated with its own device or a singleexisting device being connected to each of the three phases inalternating succession. It is also possible to transmit the data emittedby the evaluation unit 7 at the output 11 through the system to acentral control point and/or to other devices. Conversely, the centralstation can transmit information or control signals to one or moredevices via the system. By means of this, all of the devices and thevoltage sources connected to them can be shut down as needed from acentral station or status signals can be sent to the central station byall of the connected devices. The transmission takes place, for example,by means of a high-frequency modulation of the system voltage with theaid of power line modems (bi-directional power line communication).Finally, it goes without saying that the various features can also beused in combinations other than those depicted and described.

[0048] It will be understood that each of the elements described above,or two ore more together, may also find a useful application in othertypes of devices of equal-rated operation differing from the typesdescribed above.

[0049] While the invention has been illustrated and described asembodied in a device for measuring the impedance of an electrical supplysystem it is not intended to be limited to the details shown, sincevarious modifications and changes may be made without departing in anyway from the spirit of the present invention.

[0050] Without further analysis, the forgoing will so fully reveal thegist of the present invention that others can, by applying currentknowledge, readily adapt it for various applications without omittingfeatures that, from the standpoint of prior art, fairly constituteessential characteristics of the generic or specific aspects of thisinvention.

1. A method for measuring the impedance of an energy supply system at arated frequency by impressing a testing current into the system andmeasuring the resulting changes in the system current and the systemvoltage, comprising the steps of measuring said system current and saidsystem voltage during a first preselected time interval (T1) without theimpressing of said testing current and during a second preselected timeinterval (T2) with the impressing of said testing current, composingsaid testing current of at least one periodic signal with at least onetesting frequency (f1, f2) that deviates from said rated frequency,using, based on said measured system currents and system voltages, aFourier analysis to determine a resulting impedance for said testingfrequency (f1, f2) and deriving said impedance of said energy supplysystem at the rated frequency from said impedance obtained for saidtesting frequency (f1, f2).
 2. The method according to claim 1, andfurther composing said testing current of at least two periodic signalswith testing frequencies (f1, f2) that differ from the rated frequency,using said Fourier analysis to determine resulting impedances for eachtesting frequency (f1, f2), and deriving said impedance of said energysupply system through approximation based on said impedances calculatedfor said testing frequencies (f1, f2).
 3. The method according to claim1 or 2, and composing said testing current of sinusoidal signals.
 4. Themethod according to claim 2, and composing said testing current of twosignals having testing frequencies (f1, f2) which are situatedsymmetrically on either side of the rated frequency.
 5. The methodaccording to claim 4, wherein said impedance of said energy supplysystem is calculated by averaging impedances determined at said twotesting frequencies (f1, f2).
 6. The method according to one of claims2, 4 or 5, and further carrying out said Fourier analysis separately forsaid testing frequencies (f1, f2) in such a way that complex Fouriercoefficients of said system current and said system voltage arerespectively determined for said two time intervals (T1, T2), complexdifferences characteristic for changes in said system current and saidsystem voltage are derived from said complex Fourier coefficients, andsaid differences are used to calculate said impedances at said testingfrequencies (f1, f2).
 7. The method according to claim 1, wherein saidtwo time intervals (T1, T2) are at least as long as the lowest commonmultiple of a period of said system frequency and periods of saidtesting frequencies (f1, f2).
 8. The method according to claim 7,wherein said second time interval (T2) is no longer than one second. 9.The method according to one of claims 2, 4 and 5, wherein said twotesting frequencies (f1, f2) lie between 9 Hz and 121 Hz.
 10. The methodaccording to claim 1, and further letting occur said impressing of saidtesting current and said resulting changes at a same phase position assaid measurement of said system current and said system voltage withoutsaid impressing of said testing current.
 11. The method according toclaim 1, wherein a testing signal, a status signal, or a switchingsignal is derived from said impedance determined for said ratedfrequency.
 12. The method according to claim 1, wherein said impressingof said testing current occurs with the aid of a current converterconnected in parallel to said system.
 13. The method according to claim12, and further comprising the steps of using a current converter (14)to detect said system current and said system voltage and connectingsaid current converter (14) in parallel to said system.
 14. The methodaccording to claim 12 or 13, wherein a system current nominal value isgenerated in said current converter and a desired value of said testingcurrent is superposed onto said nominal value.
 15. The method accordingto claim 14, wherein said desired value of said testing current istransmitted to said current converter (14) by an independent evaluationunit (7).
 16. An apparatus for measuring the impedance of an energysupply system at a rated frequency, comprising: means (3, 14) forimpressing a testing current into said system, a measuring device (4)for measuring a system current, a system voltage, and changes in saidsystem current and said system voltage caused by the impressing of saidtesting current, and an evaluation unit (7) connected to said measuringdevice (4), wherein said system current and said system voltage aremeasured during a first preselected time interval (T1) withoutimpressing of said testing current and during a second preselected timeinterval (T2) with the impressing of said testing current, said testingcurrent is composed of at least one periodic signal with at least onetesting frequency (f1, f2) that deviates from said rated frequency, aFourier analysis is used, based on said measured system currents andsystem voltages, to determine a resulting impedance for said testingfrequency (f1, f2) and said impedance of said energy supply system atthe rated frequency is derived from said impedance obtained for saidtesting frequency (f1, f2).
 17. The apparatus according to claim 16,wherein said means (3, 14) include a controllable current source. 18.The apparatus according to claim 17, wherein said evaluation unit (7)controls said current source.
 19. The apparatus according to claim 17,wherein said controllable current source is a current converter (14)connected in parallel to said system.
 20. The apparatus according toclaim 19, wherein said measuring device (4) is at least in part anintegral component of said current converter (14).
 21. The apparatusaccording to claim 19 or 20, wherein said evaluation unit (7) is atleast in part an integral component of said current converter (14). 22.The apparatus according to claim 19, wherein said current converter (14)causes said system current to track a desired current value and saidtesting current is generated by correspondingly changing of said desiredcurrent value.
 23. The apparatus according to claim 22, wherein saiddesired value of said testing current can be generated in said currentconverter (14) and can be switched on and off in response to signalsfrom said evaluation unit (7).
 24. The apparatus according to claim 22,wherein said desired value of said testing current can be stored in amemory of said static power converter (14) and can be switched on andoff in response to signals from said evaluation unit (7).
 25. Theapparatus according to claim 16, wherein said measuring device (4) has aband-stop filter means that is matched to said rated frequency.
 26. Theappatus according to claim 16, wherein said measuring device containsband-pass filter means that are matched to said testing frequencies. 27.The apparatus according to claim 16, wherein a known testing impedance(19) is connected between said current source and said system, andwherein a controllable switch (20) is connected in parallel with saidtesting impedance.
 28. The apparatus according to claim 27, wherein saidevaluation unit (7) controls said switch (20).
 29. The apparatusaccording to claim 16 and further being provided with an individualidentification means, which can be fed into said system, and with meansfor analyzing and taking into account of those of said identifications,which have been fed into said system by other corresponding devices.